Natural products continue to be a rich source of clinical drugs for treatment of human and animal diseases.1,2 With respect to drug development, advanced understanding of their biosynthesis is significant for rational strain improvement efforts. This includes genetic manipulation (e.g. gene knock-out, knock-in, and whole gene cluster amplification) of the key biosynthetic and regulatory genes in order to increase the yield of pharmaceuticals to a desired level.3-6 Knowledge on biosynthesis is also valuable for guiding generation of novel natural product analogs as new drug candidates by metabolic engineering, mutasynthesis and allied approaches.7-11 In addition, biochemical characterization of diverse biosynthetic enzymes continues to reveal new catalytic mechanisms that inspire inventions of novel chemical and biological catalysts in organic chemistry for production of fine-chemical and medicinal agents.12,13 
Elucidation of the biosynthetic pathway of a particular natural product or a family of natural products first requires identification of the gene cluster encoding its production.14-16 Next, the combined genetic (in vivo) and biochemical characterization (in vitro) of each individual biosynthetic enzyme provides important information, including enzyme substrate specificity, co-factor requirements, and the precise order of multiple biosynthetic steps.17,18 With this information available, it becomes possible to reconstitute the entire biosynthetic pathway in a heterologous host19-21 or in a multi-component in vitro reaction.22,23 
Across all microbes, plants and animals that generate natural products, it is particularly challenging to elucidate a biosynthetic pathway completely when unprecedented steps are involved, or precedent knowledge of biosynthetic origin is limited or non-existent. Conventionally, the hunting for such enzymes catalyzing these unusual biotransformations via unexplored mechanisms depends on implementing reasonable biosynthetic principles, and the scanning of the activity of all possible candidate enzymes against all hypothetical substrates.18,24,25 Thus, the entire process can require prolonged and intensive efforts, especially for those complex natural products assembled by a large number of biosynthetic enzymes.
Due to the discovery of natural products from different microorganisms bearing the same unique structural core, but varying from one another in their tailoring groups, opportunities for facile identification of unique enzymes arise. In this scenario comparative bioinformatic analysis suggests that homologous genes can be linked to formation of a common structural core, whereas cluster-specific genes provide the basis for structural differences.26-29 Recent advances in whole genome sequencing technology have made this approach rapid and cost-effective.30-34 Thus, identification of biosynthetic gene clusters for structurally related natural products from different microorganisms has become practical for comparative analysis of these systems. Deep annotation provides adequate information to develop hypotheses regarding key gene(s) and their protein products. This in turn guides experimental strategies to explore unusual biotransformation(s) of interest using genetic and/or biochemical approaches. Although considerable information can be gleaned from biosynthetic pathway mining and annotation, putative biochemical function can only be verified by analysis of the gene product in vitro using natural or suitable model substrates.